All televisions utilize a display device to transform video information into light. This is typically accomplished through the use of electronic controls that convert the video information into control signals that operate the display device. However, display devices may vary in their size, resolution, and aspect ratio, among other characteristics. Likewise, the video information format, resolution, and aspect ratio may vary, Hence, the video information provided to a television may not correspond to the characteristics of the display device used in the television or other display device. In particular, the aspect ratio of the video information may not match the aspect ratio of the display.
This problem typically arises when video signals formatted with one aspect ratio are displayed on a television with a display device having a different aspect ratio. The aspect ratio of a television picture image is a ratio of horizontal length to vertical length, expressed in relative units. Standard video signals, such as NTSC and PAL video signals, are formatted with a 4:3 aspect ratio (i.e., 1.33 aspect ratio), whereas non-standard video signals, such as HDTV video signals, are formatted with an aspect ratio greater than the standard 4:3 aspect ratio. For example, an HDTV video signal is typically formatted with a 16:9 aspect ratio (i.e., 1.77 aspect ratio). Modern cinematographic theater movies, not made expressly for conventional television, are typically films with aspect ratios greater than 1.33, typically ranging between 1.65 and 2.35.
When standard video signals are displayed on a standard television screen (i.e., a television screen having a 1.33 aspect ratio), the picture image appears on the entire television screen. As long as these standard video signals are displayed on a standard television screen, the display device is illuminated over the entire viewing area of the display. When a non-standard video signal having a different aspect ratio, such as an HDTV video signal, is displayed on a standard television screen, either the image needs to be cropped or distorted to fit the standard screen aspect ratio, or the picture image may appear in its original aspect ratio on only a portion of the screen (e.g., on the middle horizontal region of the television screen with horizontal black bars on the respective top and bottom regions of the television screen). As a result, a region of the display that would normally be illuminated in response to a standard video signal is not illuminated in response to the non-standard video signal. Likewise, television screens having a 16:9 aspect ratio may illuminate only the central portion of the display and have black vertical bars on either side of the screen when displaying a standard video signal.
For example, referring to FIG. 2, a first display device 100 has a first screen aspect ratio of 4:3 and displays a video signal having the same aspect ratio to illuminate a region 105 comprising the entire display area of the first display device 100. In this case, the external video signal is suited to the display device. Referring to FIG. 3, a second display device 102 has a second screen aspect ratio 16:9 and displays an external video signal having the same aspect ratio to illuminate a region 107 comprising the entire display area of the second display device 102. Again, in this second case, the external video signal is suited to the second display device. However, referring to FIG. 4, in a third case if the first display device 100 receives a video signal having a different aspect ratio of 16:9, primary region 105′ is illuminated while secondary regions 104 of the display may not be illuminated. Similarly, referring to FIG. 5, in a fourth case if the second display device 102 receives an external video signal having a different aspect ratio of 4:3, primary region 107′ is illuminated while secondary regions 106 of the display may not be illuminated.
For some display devices, illumination of one portion of a display device only does not have an effect on the display device. For example liquid crystal devices use a backlight to illuminate the entire viewing area of the display even if only a portion of the display has information. In this case, the light illuminating the region of the display that has no information is blocked by the liquid crystals. For other display devices, however, illuminating one region of a display and not others for any significant period of time results in differential aging of the display pixels in such areas, such that the pixels in the illuminated area are aged and those in the dark areas are not. Differential aging can result in differential performance of a pixel, such as differential brightness. When a standard video signal is then displayed on a standard television screen on which non-standard video signals have been displayed over an extended period of time, the top and bottom horizontal regions of the television screen as illustrated in FIG. 4 may be distinctly brighter than the middle horizontal region of the television screen due to the differential aging of pixels in such areas. A similar phenomenon occurs when a standard video signal is displayed on a non-standard television screen for an extended period of time (as illustrated in FIG. 5), causing the middle vertical region of the nominal scanning area of the display to appear darker than the respective left and right vertical regions of the display. This differential aging phenomenon thus can result in visible artifacts when the display is uniformly illuminated. Most viewers will complain about this phenomenon.
This problem has been addressed for televisions using a cathode ray tube display. U.S. Pat. No. 6,359,398 B1 entitled “Method to control CRT phosphor aging” issued Mar. 19, 2002 describes methods and apparatus that are provided for equally aging a cathode ray tube (CRT). A video input terminal is coupled to the CRT and receives an external video signal. Control circuitry is provided, which detects the aspect ratio of the signal and determines whether there is a mismatch between the signal aspect ratio and an aspect ratio of a display screen in association with the CRT. If a mismatch between the signal aspect ratio and the screen aspect ratio exists, an equalization video signal is derived from the external video signal. A primary region of the CRT is illuminated in response to the external video signal, and a secondary region of the CRT, which would otherwise not be illuminated in response to the external video signal due to the mismatch between the signal aspect ratio and the screen aspect ratio, is illuminated in response to the equalization video signal. In this manner, the CRT is uniformly aged. However, the solution proposed requires the use of blocking means such as doors or covers that may be manually or automatically provided to shield the illuminated secondary regions from view when the equalization video signal is applied to the display. This solution is unlikely to be acceptable to most viewers. U.S. Pat. No. 6,369,851 entitled “Method and Apparatus to Minimize Burn Lines in a Display” issued Apr. 9, 2002 describes a method and apparatus for displaying a video signal using an edge modification signal to minimize burn lines. However, this solution only reduces the burn lines at the edges of the regions and does not address overall brightness differences between different regions.
The general problem of brightness differences due to aging has also been addressed for OLED display devices. An OLED display includes organic materials that glow in response to a current passed through the materials. Over time, the response of the organic materials to the current declines and the display becomes less bright. If one pixel of a display receives more current and emits more light over a significant period of time, that pixel of the display will not be as bright as pixels for which less current is supplied. One proposed solution is found in WO2004027744 A1 entitled “Matrix Display Device with Photosensitive Element” published Apr. 1, 2004. This disclosure describes a matrix display device that comprises an array of addressable pixels each having a display element and a control circuit for controlling the operation of the display element. The control circuit includes a charge storage capacitor and a photosensitive device coupled to the storage capacitor for regulating charge stored on the storage capacitor in accordance with light falling on the photosensitive device. However, such a solution requires a complex driving circuit for each pixel and with additional control elements.
A variety of other methods for measuring or predicting the aging of the OLED materials in displays are known in the art. Most such techniques do not compensate for the differential aging created by signals and displays having different aspect ratios. U.S. Pat. No. 6,414,661 entitled “Method And Apparatus For Calibrating Display Devices And Automatically Compensating For Loss In Their Efficiency Over Time” issued Jul. 2, 2002 to Shen et al, e.g., describes a method and associated system that compensates for long-term variations in the light-emitting efficiency of individual organic light emitting diodes (OLEDs) in an OLED display device, by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel and derives a correction coefficient that is applied to the next drive current for each pixel. Similarly, U.S. Pat. No. 6,504,565 entitled “Light-Emitting Device, Exposure Device, And Image Forming Apparatus”, issued Jan. 7, 2003 to Narita et al describes a light-emitting device which includes a light-emitting element array formed by arranging a plurality of light-emitting elements, a driving unit for driving the light-emitting element array to emit light from each of the light-emitting elements, a memory unit for storing the number of light emissions for each light-emitting element of the light-emitting element array, and a control unit for controlling the driving unit based on the information stored in the memory unit so that the amount of light emitted from each light-emitting element is held constant. US Published Patent Application No. 2002/0167474 entitled “Method Of Providing Pulse Amplitude Modulation For OLED Display Drivers” published Nov. 14, 2002 by Everitt describes a pulse width modulation driver for an organic light emitting diode display. One embodiment of a video display comprises a voltage driver for providing a selected voltage to drive an organic light emitting diode in a video display. The voltage driver may receive voltage information from a correction table that accounts for aging, column resistance, row resistance, and other diode characteristics. These methods require complex circuitry and storage techniques for every pixel, greatly increasing the complexity of the display.
Accordingly, there is a need for an improved method and apparatus for uniformly aging a display device of one aspect ratio when driven by a video signal having a different aspect ratio.